1. Field of the Invention:
The present invention relates to a variable valve timing mechanism for controlling the valve timing of at least one of an intake and an exhaust valve in a manner such that the valve has its opening and closing operation timed to a running condition of an internal combustion engine, and more particularly to the variable valve timing mechanism in which a non-constant velocity joint is used to convert a constant input angular velocity into a variable output angular velocity while the joint makes one turn.
2. Description of Related Art:
A reciprocating internal combustion engine (hereinafter referred to as the engine) is provided with an intake and an exhaust valve (hereinafter referred to, as a general term, as the "engine valve", or simply as the "valve"). Since the valve is driven in a condition in which its lift depends on a cam shape and a current phase angle (i.e., current angular position) of the cam in rotation relative to a fixed reference angular position of the cam, both a timing of each of a valve opening and a valve closing operation of the valve and the duration of valve opening of the valve also depend on the cam shape and the current angular position of the cam in rotation, provided that the duration of the valve opening is a period of time for which the valve is kept open, the duration being expressed numerically in crankshaft angle.
On the other hand, as for the intake and the exhaust valve both provided in the engine, the optimum timing of each of the valve opening and the valve closing operation and also the optimum duration of the valve opening vary in response to a current engine load and engine speed. Consequently, in order to control the timing and the duration of valve opening, various types of a so-called variable valve timing apparatus or mechanism have been proposed.
In addition to these proposed types, an additional and noted one has been also developed, in which one: a non-constant velocity joint with an eccentric mechanism is interposed between a cam and a camshaft; they (i.e., the cam and the camshaft) are parallel to but radially offset from each other, so that the cam has its axis of rotation be eccentric to that of the camshaft, which enables the cam to vary its angular velocity when the camshaft makes one turn at a constant angular velocity; consequently, it is possible for this noted one to adjust the timing and the duration of each of the valve opening and the valve closing by adjusting an amount of eccentricity of the axis of rotation in the cam of the eccentric mechanism.
Use of such non-constant velocity joint is disclosed in, for example: Japanese Patent Publication No. Sho 47-20654; Japanese Patent Laid-Open Nos. Hei 3-168309, Hei 4-183905, Hei 6-10630 and the like.
In the variable valve timing mechanism using such non-constant velocity joint, the pattern in which the timing of each of the intake and the exhaust valve varies depends on a direction of rotation, in which direction the eccentric machanism is adjusted in its eccentricity. This pattern affects the engine power output characteristics.
Consequently, there is the need for adjusting a current angular position of the eccentric mechanism in a rotational direction suitable for the engine power output characteristics.